First,
Nice speaker cabinets, respect, especially building horns.
On the Shapeoko front, your machine can and will do way better than what you’re seeing. I routinely get plywood pockets accurate to about 0.1mm in X and Y first cut without tweaking CAM.
I made a long post a while ago on the sources of deflection in the machine, and how large they are / should be;
What you’re seeing is way over what I would expect. Checking the spindle deflection under a moderate force as you’re doing (I used 25N when testing) is a very good way to get an overall reading of the machine, did you check in X and Y to see if there’s a difference? Checking for Z carriage ‘nodding’ under load in Y is also a strong indicator of where the issue lies.
I suggest, in order of effort and time, checking again with your caliper after each stage.
- Loosen all the lower V wheels on X and Y, check all the upper V wheels sit square and level on the rails, none should sit above and they should be in line above the V. Re-tension the lower wheels to tight enough that one finger can’t stop them rotating when you roll the axis along.
- Check your belt tensions, measure the tension preferably using one of the easy methods (guitar tuner etc.)
Measuring belt tension, squaring and calibration - Go round all the ‘joints’ on the machine and apply your finger pressure to the spindle in X and Y, place fingers of your other hand across the gaps between moving Z and fixed Z, Z and X beam, X beam and Y beams, try to feel where the movement is actually occurring.
- Do the magic marker over the pulley / shaft thing so that you know if any of your stepper pulleys is starting to slip, don’t wait for it to happen, threadlock the grub screws and put the mark on now
Hope that gets you started.